The present invention relates to burnable absorbers (poisons) used in light water reactors. It is especially concerned with those burnable absorbers which are used in reactors as discrete poison rods (i.e., not mixed with fuel in a fuel rod).
In the past, commercial light water reactors have utilized burnable poisons such as boron compounds and gadolinia to extend the fuel cycle by allowing higher levels of U.sup.235 to be present at the beginning of the fuel cycle. In some designs, gadolinia has been mixed directly with the UO.sub.2 to form fuel elements containing pellets composed of UO.sub.2 and Gd.sub.2 O.sub.3. In other designs, the fuel pellets may be coated with a boron compounds, such as ZrB.sub.2.
In addition to the foregoing designs in which the fuel rods contained both a fissile material and a burnable absorber, the burnable absorber may also be dissolved in the coolant and/or be present in separate, stationary or mobile, (non-fueled) burnable absorber elements.
Prior designs of burnable absorber elements (see, for example, U.S. Pat. Nos. 3,510,398 and 4,342,722, which are hereby incorporated by reference in their entirety) have included a hermetically sealed zirconium alloy or stainless steel tubular rod containing a tubular member of borosilicate glass as the burnable absorber. Concentrically inside of the borosilicate glass was a smaller diameter zirconium alloy or stainless steel tube which provided structural support for the borosilicate glass. The remainder of the interior of the rod was filled with a gas, such as helium.
An improvement on the foregoing burnable absorber element has been marketed by the Westinghouse Electric Corporation and is known as a WABA (Wet Annular Burnable Absorber) rod. The WABA design includes a pair of concentrically disposed zirconium alloy tubes having an outer diameter and a length essentially the same as a fuel rod. The similarity in size permits the WABA rod to be statically positioned in aligned fuel assembly grid cells in the same manner as fuel rod or to be used as a movable poison element which may be positioned in a thimble guide tube of a fuel assembly. Annular burnable poison pellets containing B4C are disposed in the narrow annular space between the concentric tubes, which has been sealed shut by end plugs welded onto the tube ends. However, the hollow portion of the inner tube is not plugged and therefore permits the unimpeded flow of aqueous coolant upwardly through the inner tube during the time of operation in the water reactor. Examples of WABA rods are described in U.S. Pat. Nos. 4,460,540 and 4,474,728. These patents are hereby incorporated by reference in their entirety.